Many valves used in industrial refrigeration are pilot operated. Pilot operated valves function by applying pressure to a piston in the valve which either pushes the valve open or shut. As long as the pressure is maintained on the piston the valve stays in the open or shut position. The source of the pilot gas or liquid can be either from the line in which the valve is installed, or an external source. The supply of the pilot gas or liquid is typically controlled by a smaller solenoid valve which is either in an on or off state, allowing pilot flow to the piston or not.
Often times, when the valve is first activated the forces used to actuate the valve are by far greater than those restricting the movement of the valve. This is because the pressure of the pilot gas or liquid used to actuate the valve is much greater than the pressure of the liquid or gas restricting the movement of the valve. It is not uncommon for there to be as much as 200 psi difference between the two. This high pressure difference across the valve, coupled with the difference in size between the large actuating piston, and the smaller size of the plug which the restricting pressures act upon causes the valve to either slam shut or fly open with great force. Valves opening and closing with a large amount of force causes pipes and hangers to shake violently. This places large amounts of stress on the piping system and supporting structures. End users do many different things to prevent this, added bracing, and special sequencing of the pilot source gas to name a few.
This force can be controlled by restricting the amount of pilot gas or liquid which is applied to the actuating piston with a fixed orifice. This technique works for the initial movement of the valve, but the problem arises after the pressures start to balance within the valve. Typically, valves are required to stay open or shut with as little as 2-5 psi difference within the valve. By restricting the pilot flow to slow the initial closing there is not enough sustained pilot flow to hold the valve open or closed after the pressures start to balance.